Various solutions exist for ensuring the detachable mounting of clip-ons on spectacle frames. Thus, magnetic-type solutions are described in U.S. Pat. No. 5,737,054, U.S. Pat. No. 6,547,703 and U.S. Pat. No. 6,412,942. Mechanical-type solutions are described in U.S. Pat. No. 6,474,810, U.S. Pat. No. 5,774,200, U.S. Pat. No. 5,724,118, U.S. Pat. No. 5,694,192, U.S. Pat. No. 5,493,348 and U.S. Pat. No. 5,790,230.
Clip-ons have been proposed in order to meet the needs of particular presbyope wearers, such as radiologists, aircraft pilots or others. These wearers need zones of vision different from the standard zones used for progressive lenses—near vision at the bottom of the lens, far vision at the top of the lens. For example, aircraft pilots need a near vision zone at the top of the lens, in order to be able to look at dials situated high up in the cabin.
WO-A-01 81987 proposes equipment intended for presbyope wearers, needing to look at near or intermediate points in the object space through the part of progressive or multifocal lenses which corresponds to intermediate vision or far vision. This document proposes to add to a spectacle frame a support with additional positive power lenses, extending over the upper part of the spectacle lenses. The additional lenses are bifocal or have a power variation. The additional lenses given by way of example are defined solely by their power in the bottom part and in the top part. A first example relates to an additional lens having an additional power of one dioptre in the lower part and an additional power of 0.25 dioptre in the upper part. A second example relates to an additional lens having an additional power of 0.25 dioptre in the lower part and an additional power of one dioptre in the upper part. The additional lenses and their shape have the effect of generating a significant discontinuity in the wearer's optical function at the lower part of the lens. A simulation carried out with a Varilux Panamic type progressive lens with nil base and with addition of 2 dioptres produces the following results. For an additional monofocal lens with a power of one dioptre, the optical function of the Varilux Panamic and additional lens assembly exhibits, along the meridian, a sudden discontinuity of more than one dioptre, when the gaze passes on the lower edge of the additional lens. For an additional lens with power varying from one dioptre in the upper part to 0.25 dioptre in the lower part, the optical function of the Varilux Panamic and additional lens assembly exhibits, along the meridian, a sudden discontinuity of the order of 0.75 dioptre, when the gaze passes on the lower edge of the additional lens. The simulation shows moreover that the peripheral aberrations of the clip-on and the lens unfortunately combine in the optical zone modified by the clip-on. The peripheral aberrations of the assembly are therefore more pronounced than the peripheral aberrations of the initial lens.
U.S. Pat. No. 6,027,214 proposes clip-ons with local correction zones, dedicated to particular applications. This document does not specify the nature of the correction zones. U.S. Pat. No. 5,880,805 proposes a clip-on the upper two-thirds of which are clear (zero power) and the lower third of which has a power of one dioptre or more. U.S. Pat. No. 6,244,705 proposes a clip-on extending over the upper part of the lens in the frame, with a constant power value. U.S. Pat. No. 5,598,232 proposes a device making it possible to superimpose on the near vision zone of a bifocal lens a lens shaped like this near vision zone, of negative power; it is thus possible to pass from a bifocal lens to a lens having a uniform power suitable for far vision.
These different documents identify the particular problems of certain presbyope wearers. However, they only propose solutions based on somewhat unsatisfactory approximations. In fact, in the superimposing of the clip-on and the lens, both the clip-on and the lens are considered as thin lenses. Moreover, for the calculation, these clip-ons do not take account of the optical characteristics formed by the “initial lens”, such as for example the distribution of the power and the aberrations which constitute the optical design. These clip-ons therefore approximate to the resulting optics of the lens and clip-on assembly. In terms of comfort for the wearer, these solutions are not satisfactory.
Moreover, in order to establish an optical prescription, ophthalmologists generally use combinations of lenses which are superimposed in front of the wearer.